Shedding light on WDM-PON

Although fiber to the home (FTTH) is now considered mature, the industry is still bringing new evolutions of the technology to the forefront. It is critical that designers, planners, and managers know what these evolutions are and how to migrate from a legacy or next generation fiber to the user (FTTx) network.

FTTx has grown from legacy B-PON, G-PON, and EPON to next-generation 10 Gigabit systems. Third-generation FTTx uses wavelength division multiplexing (WDM) technology to increase data rates to 40 Gigabits per second as well as coexist with current systems.  Interestingly, the technology behind WDM has been around for decades, and it can provide a solution to fiber exhaust or growing bandwidth demands.

All passive optical network (PON) systems require an optical splitter to link an optical line terminal (OLT) to up to 64 optical network terminals (ONTs) using only one fiber. PON systems also require installation of a standard single-mode fiber from the service provider to each subscriber. When WDM is combined with a PON system, network capacity can be expanded as needed, or each subscriber can have their own dedicated wavelength.

Legacy systems operating at 1490 nanometers downstream and 1290 to 1360 nanometers upstream offered transmission rates as high as 2.5 Gigabits per second, and next-generation NG‑PON systems improved on these data rates. By operating at 1577 nanometers downstream and 1260 to 1280 nanometers upstream, the data rate could reach as high as 10 Gigabits per second, and coexistence with legacy systems became a possibility.

The ITU‑T G.989 recommendation is now solidifying WDM-PON as the third generation of PON solutions. With an aggregate transmission data rate up to 40 Gigabits per second, WDM‑PON maintains coexistence with both legacy and NG-PON systems. Its channel spacing is based on dense WDM spectrums: 100 GHz for downstream transmission and either 50/100 or 200 GHz for upstream wavelength channel spacing. As in dense DWDM systems, optical filters are required. Depending upon the application, a filter could be placed at the service provider’s hub, in the outside plant, or at the ONT using tunable optics.

The ITU recommendation proposes two options for optical transmission, both of which use new wavelengths for coexistence with legacy and NG‑PON systems.

  • Option 1:  This method is a combination of time and wavelength division multiplexing known as TWDM. TWDM systems use either legacy splitters (power splitting) or — in the case of greenfield installations — a wave splitter as shown in Figure 1. The ITU recommends the optical spectrum of 1524 to 1544 nanometers for upstream TWDM transmission and 1596 to 1603 nanometers for downstream TWDM transmission.

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Figure 1. Examples of flexible deployment options for coexistence and greenfields

 
  • Option 2:  This is a dedicated point-to-point (P2P) WDM-PON initially focused for dedicated businesses and back haul services. Unused spectrum from 1603 to 1625 nanometers can be used for P2P WDM‑PON systems with coexistence or between 1524 to 1625 nanometers for those without coexistence.
Unlike PON systems, a P2P WDM-PON system can span distances greater than 20 km. This is due to the removal of the optical splitter with its 15‑18 dB of optical attenuation. However, as these WDM systems operate “out of band” of the wavelengths used in older PON systems, the fiber link could either pass through an optical splitter or bypass the splitter when necessary to minimize attenuation for longer spans.

What does all of this mean for those who install, test, and troubleshoot WDM‑PON systems?

Due to the longer wavelengths in use, the sensitivity of macrobends is critical to consider when working around splice trays and other fiber/cable management products. While installing G.657 bend-insensitive single-mode fiber would mitigate this effect, it is not safe to assume that previously-installed fibers also are bend insensitive, as may be the case with most legacy systems. Testing installations with an OTDR at 1625 nanometers would be recommended whenever possible to ensure any macrobending losses do not impact the system performance. New test equipment — such as optical power meters, OTDRs, and wavelength meters — are an essential part of identifying the specific wavelengths in use.

 

Article originally published in ISE Magazine’s May 2016 issue